Abstract
Conventional parallel stability control methods for inverters primarily focus on resonant control but struggle to regulate current feedback through virtual impedance, leading to reduced parallel stability. This study proposes a novel stability control strategy for high photovoltaic energy storage inverters. By analyzing circulating current characteristics during parallel operation and designing seamless grid-connected/off-grid switching logic, the method ensures stable voltage and phase synchronization. Experimental results demonstrate superior performance compared to existing approaches, validating its practicality for real-world applications.
1. Introduction
Photovoltaic energy storage systems integrate solar generation with storage devices to provide stable grid power. The parallel operation of energy storage inverters enhances system capacity but introduces stability challenges due to voltage/current fluctuations. Existing methods like adaptive current prediction models [1] and impedance reshaping [2] inadequately address multi-inverter interactions under variable conditions. Our methodology resolves these limitations through dynamic circulating current suppression and mode-switching optimization.
2. Methodology
2.1 Circulating Current Analysis
For n parallel-connected energy storage inverters, stable operation requires:
$$
\begin{cases}
E_1 = E_2 = \cdots = E_n \\
f_1 = f_2 = \cdots = f_n \\
\phi_1 = \phi_2 = \cdots = \phi_n
\end{cases}
$$
where \( E_i \), \( f_i \), and \( \phi_i \) represent voltage magnitude, frequency, and phase of the \( i \)-th inverter. Circulating current \( I_{cir} \) is derived as:
$$
I_{cir} = \frac{E_i \angle \phi_i – U_{load}}{Z_i}
$$
Here, \( U_{load} \) denotes load voltage, and \( Z_i \) is the equivalent impedance. Impedance mismatch (\( Z_i \neq Z_j \)) creates clockwise or counterclockwise circulating currents, quantified through harmonic power analysis.
2.2 Seamless Mode Transition Control
The voltage output matrix during grid-off-grid transitions is:
$$
\begin{bmatrix}
U_a \\
U_b \\
U_c
\end{bmatrix}
=
\begin{bmatrix}
U_i \cos \theta \\
U_i \cos \left( \theta – \frac{2\pi}{3} \right) \\
U_i \cos \left( \theta + \frac{2\pi}{3} \right)
\end{bmatrix}
$$
where \( U_{a,b,c} \) are three-phase voltages, and \( \theta \) is the phase angle. Autonomous switching prioritizes voltage stability when grid abnormalities (\( U_a = U_b = U_c \)) occur.
3. Experimental Validation
3.1 Setup
Three 40kW energy storage inverters were tested under unbalanced DC sources and line impedances. Key parameters include:
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| DC Voltage (Ud1) | 800 V | Filter Inductance (L) | 2 mH |
| DC Voltage (Ud2) | 700 V | Filter Capacitance (C) | 10 μF |
| DC Voltage (Ud3) | 600 V | Line Impedance (RL1) | 0.1+j0.4 Ω |
| Grid Voltage (Ug) | 380 V | Load Power (P/Q) | 10 kW/5 kVar |
3.2 Results
The proposed method achieved 97.9% stability success rate with 0.76s average response time, outperforming conventional approaches:
| Method | Success Rate (%) | Response Time (s) |
|---|---|---|
| Adaptive Current Prediction [1] | 77.2 | 1.48 |
| Impedance Reshaping [2] | 87.1 | 2.75 |
| Proposed Method | 97.9 | 0.76 |
Voltage/current oscillations remained within ±500 V and ±200 A respectively, demonstrating effective suppression of circulating currents during parallel operation.
4. Conclusion
This study advances energy storage inverter stability through two innovations: 1) Dynamic circulating current suppression via impedance-phase synchronization, and 2) Grid-off-grid transition control using voltage matrix modulation. The method’s scalability and robustness make it suitable for high-power photovoltaic storage applications requiring multi-inverter coordination. Future work will optimize the algorithm for larger-scale (>10 MW) hybrid renewable systems.